Acoustic device



June 30, 1931. E. c. WENTE ACOUSTIC DEVICE Original Filed April l, 1925996 :2: \5 SQ GE 20:

2 umb aha 2-1 5 radius freq F 1 mum"? Patented June 30, 1931 UNITEDSTATES v PATENT OFFICE EDWARD C. WENTE, OF NEW YORTK, N. Y., ASSIGNOR,IBY MESNE ASSIGNMENTS, TO WESTERN ELECTRIC COMPANY, INCORPORATED, ACORPORATION OF NEW YORK .ecous'rrc DEVICE Application filed April 1,1925, Serial No. 19,793. Renewed September 11, 1930.

aims to provide means capable of faithfully reproducing complex waves,and especially to provide a loud speaking telephone receiver capable offaithfully reproducingelectrical "ariations' as sound such as speech-ormusic.

The invention will be described hereinafter with especial reference tosuch a receiver. In

accordance with such application of the invention, an inflexiblediaphragm is so mounted and so actuated, as set forth hereinafter, thatthe ratio of the power output in the form of sound to the electricalpower input due to the current corresponding to the sound to bereproduced, is substantially the same for all frequencies in the rangeof frequencies to be reproduced.

Fig. 1 Of the accompanying drawings is a diagram, and Fig. 2 a graph,for facilitating explanation of the invention; Fig. 3 is a sectionalelevation of an embodiment of the invention; Fig. 4 is a top plan of adetail of Fig. 3, partly broken away; and Fig. 5 is a fragmentarysectional perspective view of the device of Fig. 3.

Before describing the structure of Figs. 3 to 5 in detail, it will bewell to set forth certain theoretical considerations regarding an horn.

When a body vibrates in an elastic medium such as air, the power that isdeveloped in the form ofsound waves may be expressed by P =p '0 where Pis the power,

v is the velocity with which the body vibrates,

' plane 8, as shown in Fig. 1, vibrates as a unit perpendicularly to itsplane, p will vary after the manner shown in the curve A of Fig. 2, asmay be seen if values of the radiation resistance per unit area. of thediaphragm, for values of the product of diaphragm radius times frequencywhich lie between and 10 are calculated (all in c. g. s. units) fromEquation 10, page 164, section 302 volume 2 of Rayleighs Theory of Sound(published by MacMillan and Company, London, 1894),

and plotted as in Fig. Here R is the radiusof the disc, f is thefrequency, and

the ordinate. is the radiation resistance per unit area of the disc. Forvalues of Rf up to 10" c. g. s. units, the quantity value of Rf for adisc of given radius the radiation resistance would vary as the square'of the frequency, e. g. if we made R=4 cm.,

we would have from 0 to 2500 p. p. s. or p K and P=Kf o up to 2500 p. p.s. electromechanical loud speaker without a In any practical case such adisc would have to have some .form of elastic support. This would meanthat the disc, which has a certain mass, would have a resonant frequencyof some finite value. Above this resonant frequency the velocity of thedisc is given, to a close approximation by This equation shows that, P.the sound developed by the disc would be independent of frequency for aconstant applied force. If

now" we apply the force F by electrical means in such a way that F isproportional to the current, I, we shall have the ratio of sound outputto electrical power input (which is roportional to P) thesame for allfreuencies between resonance and 2500 p. p s.

Above 2500 pp. s p varies comparatively little withfrequency. At 5000 p.sfthe velocity of motion of the disc under t e assumed condition wouldhave one half the value it has at 2500 p. p. s. T From 2500 to 50003).p. s.

the sound output would, therefore, rop 6 .T. U. and up to 10,000 p.p.'s. another 6 T. U.

(The abbreviationT. U. is. for the transmission unit discussed in thefollowing articles:

The Transmission Unit, by R. V. L. Hart ley, in ElectricalCommunication, July, 1924,

published by the International Western Elec-' tric 00., Inez, New York;and The Transmission Unit and Telephone Transmission Systems, by Martin,and Practical Application of the Transmission Unit by C. W. Smith, bothin the .Bell System Technical Journal, July, 1924, published by the 5American Telephone and Telegraph Co., 's variationis not excessive NewYork.)- T for this large expanse of frequency,

- In practice it might not be advisable to use the disc in a plane ofvery largeextent as so was assumed in the preceding discusion. If

the back of the disc is closed oil so as to prevent radiation from theside, the sound outut will be reduced by 6 TAU. at the lower equenciesby the removal of the infinite s5 plane. The omission of the infinite"plane thus in part offsets the drop at the higher frequencies'becauseof the fact that there p approaches a constant value. A receiver con-,structed as described below renders it pos- 40 sible to transmit a bandof frequencies extending from, say 100 to 7000 cycles per sec-' ond,with the transmimionloss varyin less than 6 T. U. throughout suchfrequency and,

-. w, and at the same time, to maintam the diam- 5 eter of the diaphragmlarge enough to economically obtaln satisfactory value of sound output.

g0omtruction of the receiver To operate on the principles just setforth, a receiver should-preferably satis the following conditions: Thedisc or diaphragm should be constructed so that it will move 'as,

co the disc should-be comp etely shut oil from the outside air; theforce on the diaphragm should bepr'oportional to the current input; theimpedance of the receiver winding should not va greatly with frequency;and the disc shoul be mounted so that it is .free

to execute large amplitudes of motion, in

lar e'volume.

Y ne'construction. adapted tomeet these conditions is shown. in Figs. 3to 5. In that construction, the diaphragm 10 comprises a circularcentral portion 11 bulging in one direction, and an annular rim 12flarmg from the periphery-of the central portion in that direction, thediaphragm thus having a W-s'haped cross section, as indicated in Fig. 3.The central portion 11 is preferably dome-shaped, as shown, since astructure of this shape is speciall adapted to move as a unit up to veryhigh requencies when driven from its base as described hereinafter.However, the portion 11 may be of any shape, for instance conical,suitable for obtaining the requisite stifiness, and may, if desired, beannularly or radially corrugated to increase its stiffness. For the sakeof rigidity, the rim or outer portion 12 of the diaphragm 10 order toobtain a sound output of sufliciently is preferably frusto-conical,asindicated in Figs. 3 and 5. However the rim 12 may be of any suitableshape, for instance, downwardl concave and upwardly convex, suitable or(obtaining the stiffness requisite to enable. the rim to e vibrated as aunit up to 'veryjhigh frequencies when driven from its inner peripheryas described hereinafter. 'If desired, the rigidity ofthe rim 12 may beincreased by annular or radial corrugations, not shown. Preferably, thediaphragm 10 is of thin light metal, for instance duralumin -oraluminum. Its diameter should be sufiiciently small to insure thedesired degree ofv uniformity oftransmission efliciency over the freuency range to be transmitted. The mass 0 the diaphragm should besufficiently small to enable the receiver-to have the desiredsensitivity. Preferably the diameter should be as small as 5 inches, andthe mass as small as 5 gr. v

v The diaphragm is so mounted as to close an opening in a wall 15 of acasing 16 which closes off one side of the diaphragm from thesurrounding air. The magnet 18, which is shown as an electromagnet butma if desired, be a permanent magnet, is enc osed in the casing 16 andrigidly connected to the casing by an suitable means such, for instance,as ro s 19. As shown, the magnet comprises a winding orcoil '20surrounded by a. paramagnetic fi-ux 'path21 havin an annular air-gap 22therein at the end o the hollow core 23adjacent to, the diaphragm 10.

Terminals 24, only one of which is shown,-

are provided forcoil 20, so that the coil may be energized by directcurrent.

The r.'

1V1I1g system for the diaphragm is shown as of the movable coil type,the coil 25 for receiving the telephonic current being situated in theradial magnetic field in the aingap 22. This type of drivin unit has aorcefactor, and the impe ance oi large posts 26, only one of which isshown, form terminals for the coil 25. This coil is rigidly connected tothe diaphragm at the junction of the'central portion 11 and the rim 12,

as shown in Figs. and 3, so that the elements 11, 12, and form a rigidunit. A ring preferably of very thin metal, for instanceduralumin oraluminum extends outwardly from the periphery of the rim 12, in theplane of that periphery. Bin 30 is supported from the wall 15. Preferaly the outer periphery of the ring engages the outer cylindrical wall31' of'an annular'recess 31 in the inner edge of the wall, and above andbelow the ring 30 the recess is filled with paper rings 32, such as areused for so-called book damping. If desired, the pa hold ring 30 tigltly near its outer periphery and loosely near its inner edge, forinstance by tapering (not shown) the recess so that it is narrower nearwall 31' than near the periphery of the,- diaphragm. The paper ringsoffer only slight opposition to movement of the diaphragm 10 in thedirection perpendicular to the plane ofthe ring 30, and thus permitsubstantially free vibration of the diaphragm as a unit in thatdirection, with lar e amplitude, for instance a double amplitu e of twomillimeters, while at the same time maintaining the coil 25in properposition for vibration in the air-gap 22. A member 34 preferably in theform of an annular spider comprising a hub portion 35, a rim 36, andradial spokes 37 connecting the hub and the rim, is rigidly connected toand extends outwardly from'the junction of the central portion 11 andthe rim 12 of the diaphragm 10. Preferably, the hub of the spider 34 isintegral with the central portion 11 of the diaphra m, as indicatedin'Fig. 5, and is of thin ralumin or aluminum, with the rim annularlycorrugated as at 38, the hub annularly corrugated as at 39, and thespokes radially corrugated as at 40 (see Fig. 4). All of thesecorrugations open into the plane of the spokes.

- air-gap. The

The rim of the spider is supported from the structure of magnet 18 andcasin 16, preferably by being clamped in the inner edge of a ring rigidwith magnet 18. The

spider 34, as well as the ring 30, supports the diaphragm and movingcoil for substantial- 1y free vibration of the spider, with largeamplitude, while maintaining the coil in proper position in the annularcorrugations 38 and 39 allow the width of the spider 34 to increase whenthe spider is flexed from its normal position and theradial-corrugations 40 tend to prevent undesirable'vibrations in thespider.

In order to keep the resonant frequency of er rings may be caused tolight metal, for instance duperpendicularly to the plane the diaphragmlow, refdrabl as low as 100 cycles 'per second, t e cham r formed bycasing 16 is of such size that the volume of 'air in the chamber islarge compared to the maximum volumetric displacement executed by thediaphragm during its normal 0 era-- tion. For instance, with a diaphraing a diameter of the order of 3% inc es, and a magnet having a diameterof the order of 6 inches and 9. Ion h of 4 inches, the casing 16 may bea box-1i e casing having dimensions of the order of 12"by 12-b 8' indepth. To reduce standing waves wit in the casing, preferably the holethrough the magnet core is flared and the portion of the chamber notoccupied by the magnet is substantially filled with some dampin materialsuch as loose wool or down 51 as indicated in the drawing.

To reduce eddy currents, slits 50, parallel to the axis of core 23, areprovided in the end of the core adjacent coil 25'.

The aper rings 32 effectively prevent passage 0 air or sound wavesaround the edge of the diaphragm 10, from the outside to the inside, orvice verse, of the chamber formed by casing 16, and yet, as noted above,permit substantially free vibration of the diaphragm, as a whole,through large amplitudes, at the frequencies to be reproduced. The paperrings also prevent sound originating at one face of the diaphragm fromimpinging on one face of the ring 30 and being transmitted through thatrin to the air on the other side of that ring. The paper rings alsoprevent resonant vibrations of the ring 30 itself.

In the operation of the device as a telephone receiver, coil 20 isenergized by direct current, to create a steady, radial electromagneticflux across ga 22; and the binding posts 26 (only one 0 which is shown)of coil 25 are connected to the source (not shown) of telephoniccurrent, the variations of which are to be reproduced as sound. Thediaphra m and the coil 25 vibrate as a whole, in accor ance with thevariations in the latter current, the diaphragm giving off the soundcorresponding to those current variations. The device is of coursereversible. That is, sound waves impinging on the diaphragm will causethe device to function as a transmitter, generatin spondingly varying E.F.

By properly fixing the extent of the wall 15 beyond the periphery of thediaphragm 10, the effect of the wall in increasing the sound output ofthe receiver at low frequencies more than at high frequencies, can bemade such in coil 25 a corre- I quency range upto 3500 cycles per secondor 5500 cycles per second in these two cases, respectively. The specificfre- -quency and radius values here mentioned are merely illustrative.As a further example, if the frequency range which it be desired totransmit with substantially uniform efiiciency extends from 5000 cyclesdown to quite low frequencies, and the radius of the diaphragm be 4 cm.,then it may be desirable to have the extent of the wall negligiblysmall, in accordance with the principles hereinbefore explained.

Distortion due foam-linear response Not only may the receiver of thisinvention be made to have practically a uniform response over the Wholefrequency range to be reproduced, as for instance the range offrequencies of importance in music, but it is, moreover, free fromanother type of distortion introduced by most loud speakers. This lattertype of distortion'is producedif at a given frequency the displacementis not proportional to force. Since this receiver operates above theresonant frequency of the diaphragm, the amplitude,

' if the diaphragm moves as a unit m, the effective mass of thediaphragm, is constant and a and F are proportional.

The broad features involving the general principles disclosed herein maybe embodied in many organizations widely different from thosespecifically shown and described, Without departing from the spirit ofthe invention defined in the appended claims.

What is claimed is:

1. An acoustic device comprising 9. diaphragm, an air chamber for whichsaid diaphragm forms a closure member, the air inclosed in said chamberforming a cushion for said diaphragm, and means mounting said diaphragmon said air chamber, the volume of said inclosed air being sufficientlylarge and the stiffness of said mounting means suffici ently small inmagnitude to maintain the resonance frequency of the diaphragm below thefrequency range of importancein music.

2. An acoustic device comprising a direct actin diaphragm, an airchamber for which said 'aphragm forms a closrre member, the air inclosedin said chamber forming a cash ion for said diaphragm, and meansmounting said diaphragm on said air chamber, the volume of said inclosedair bein sufficiently large and the stiffness of said mounting meanssufficiently small in magnitude to maintain the resonance frequency ofthe diaphragm below the frequency range of importance in music. g

3. An acoustic device comprising a plunger type diaphragm having aW-shaped cross sectionand an air chamber for which said diaphragmforms'a closure member, said chamher being too large to cause resonanceof the diaphragm above cycles per second.

4, An acoustic device comprisinga plunger type diaphragm havin itsnatural frequency below the frequency range of importance in speech, anair chamber for which said diaphragm forms a closure member, and adriving coil rigid with said diaphragm, said chamber being too large, incomparison to the maximum volumetric displacement executed by saiddiaphragm in normal operation, to cause resonance of the diaphragm insaid frequency range.

5. A device comprising a diaphragm having a natural frequency ofvibration below the range of frequencies of importance in speech andhaving a radiation resistance varying substantially as the square of thefrequency of vibration of the diaphragm over said frequency range,'andmeans responsive to current of speech frequencies for driving saiddiaphragm with a force proportional to said current.

6. An acoustic device comprising a diaphragm having a natural frequencyof vibration below the range of frequencies of importance in music andhaving a radiation resistance varying substantially as the square of thefrequency of vibration of the diaphragm over said frequency range, andmeans responsive to current of frequencies of said range for drivingsaid diaphragm With a forceproportional to said current.

7. A device comprising a diaphragm having a natural frequency ofvibration below the range of frequencies of importance in speech andhaving a radiation resistance varying substantially as the square of thefrequency of vibration of the, diaphragm over said frequency range, andmeans responsive to power variations representing sound waves forcausing said diaphragm to generate sound wave power proportional inmagnitude to said first mentioned power variations.

8. A device comprising means responsive to electromotive force, of thefrequencies of the range important in music, for generating mechanicalforce substantially proportional to the electrical current delivered tosaid I phragm having a natural frequency of vibration below the range offrequencies of importance in speech and having a radiation resistancevarying substantially as the square of the frequency of vibration ofsaid diaphragm over said frequency range, means responsive to current ofspeech frequencies for driving said diaphragm with a force proportionalto said current, and a wall extending outwardly from, and substantiallyin the plane of, the periphery of said diaphragm, and increasing thesound power produced by said diaphragm.

' 10. A device comprising a piston type, direct acting diaphragm havinga natural frequenc of vibration below the range of frequencies ofimportance in music, means responsive to current of the frequencies ofsaid range for driving said diaphragm with a force proportional to saidcurrent, and a wall extending outwardly from, and substantial- 1y in theplane of, the periphery of said diaphragm, a distance of such magnitude,with regard to the diameter of the diaphragm, as

to substantially equalize the sound power output, per unit of saidforce, over said range.

11. An acoustic device comprising a diaphragm, an air chamber for whichsaid diaphragm forms a closure member, the air 1nclosed in said chamberforming a cushion for said diaphragm, and means mounting said diaphragmon said air chamber, the volume of said inclosed air being sufiicientlylarge and the stiffness of said mounting means sufficiently small inmagnitude to maintain the resonance frequency of the diaphragm below thefrequency range of importance in music, said first mentioned meanscomprising book damping means for said diaphragm.

12. A device comprising a diaphragm having a W-shaped cross section,book damping means for said diaphragm, and driving means for saiddiaphragm attached to the symmetrical cusps of said cross section.

13. A device comprising a diaphragm provided with book damping means andhaving a natural frequency of vibration below the range of frequenciesof importance in speech and having a radiation resistance varyingsubstantially as the square of the frequency of vibration of thediaphragm over said frequency range, and means responsive to current ofspeech frequencies for driving said diaphragm with a force proportionalto said current.

14. An acoustic device comprising an inflexible diaphragm having anatural frequency below a range of frequencies to be reproduced, meansclosing ofi one side of said diaphragm from the surroundingsoundpropagating medium, means supporting said diaphragm forsubstantially free vibration,as a unit with large amplitude, atthefrequencies of said range, and means responsive to current of thefrequencies of said range for driving said diaphragm with a forceproportional to said current. I 15. A loud speaking telephone receivercomprising a diaphragm having a forwardly bulging central portion and arim flaring backwardly from the bulging portion, means closing one sideof said diaphragm off from the surrounding air and mounting saiddiaphragm for vibration as a whole, at the frequencies of importance inspeech, and means attached to said diaphragm at the junction of saidbulging portion and said rim and re sponsive to current of saidfrequencies for drivin said diaphragm. 16. device comprising aparamagnetic body forming a flux path with an air gap therein, aninflexible diaphragm for vibration as a whole, a coil, means rigidlyconnecting said coil to said diaphragm, and means supporting saiddiaphragm for vibration as a whole, at the frequencies of importance inspeech, with said coil centered in said gap, said means includingannularly and radially corrugated means connected to said diaphragm andextending outwardly from said diaphragm, with respect to the axis ofsaid diaphragm in the direction of said vibration. 1 A loud speakingreceiver comprising a paramagnetic body forming a flux path with an airap therein, a diaphragm having a forwar lybulging central portion and arim flaring backwardly from said bulging portion, a coil, means rigidlyconnecting said coil to the junction of said central portion and saidrim, an annular web extending outwardly fromthe periphery of saiddiaphragm, an annular spider extending outwardly from the periphery ofsaid central portion, and means, including said web and said spider forsupporting said diaphragm and said coil for vibration with said coilcentered in said air gap.

18. An acoustic device comprising a diaphragm, an air chamber for whichsaid diaphragm forms a closure member, the air in closed in said chamberforming a cushion for said diaphragm, loose fibrous material occupyingthe major portion of said air chamber, an means mounting said diaphragmon said air chamber, the volume of said inclosed air being sufiicientlylarge and the stiffness of said mounting means sufficiently small inmagnitude to maintain the resonance frequency of the diaphragm below thefrequency range of importance in music.

19. A loud speaking receiver comprising a piston type, direct actingdiaphragm having a natural frequency of vibration below of said current,as high as of the maximum value of said ratio for said frequency range.g y 20. An acoustic device comprising a direct acting, pistontypediaphragm, an air chamher for which said diaphragm forms a 010--sure member, 'fibrous material in said air chamber for reducing standingwaves therein, the air enclosed in said chamber forming a cushion forsaid diaphragm,-and means mounting said diaphragm on said air chamber'for vibration of said diaphragm as a whole, the volume of said enclosedair being sufiiciently large and the stifiness of said mounting meanssutliciently smallin magniture to maintain the resonant frequency of thediaphragm below one hundred cycles per second, said diaphragm having adiameter sufliciently small to render said device capable oftransmitting a band of frequencies ex-' tending from one hundredcycles'to seven thousand cycles per second with a maximum variation intransmission loss, with frequency, of six transmission units.

21. A loud speaking receiver comprising a piston type, direct actingdiaphragm having a natural frequency of vibration below one hundredcycles per second; means closing one side of said diaphragm oil from thesurrounding air, and means responsive to current of 40 the frequenciesof importance in music for driving said diaphragm, said diaphragm havinga diameter sufliciently small to render said receiver capable oftransmitting a band of frequencies'having a width of approximately seventhousand cycles per second with a maximum variation of transmission lossas low as six transmission units.

22. Aloud speaking receiver comprising a' piston type, direct actingdiaphragm, means closing one side of said diaphragm ofi from the.surrounding air, and means responsive to current of frequencies of therange of'imn the order of one hundred cycles per second to a frequencyof the orderof six thousand cycles per second. a 23; An acoustic devicecomprising a diaphragm, a coil secured thereto for driving thediaphragmfa pair ofwencentric lepieces. separated by a gap in which saicoil center mam ' for centering said coil in said gap, said means a pairof concentric pole-pieces rtion being'expansible when'said coil is movedfrom its normal position to enable the coil to move axially. I

25. An acoustic device comprising a rigid diaphragm, means atther'iphery of the diaphragm for supporting it so that it is free tomove dil phragm comprising pole-pieces separatedby a narrow ga and anactuatin to said diap agm, and a spi er-like-memher for preventinglateral movement of said coil in said gap, said member being rovided.with annular corru tions to allow t e width means for actuating saiddiaP coil secured of the member to increase when it is flexed form itsnormal 'podtion and with radial corrugationsto prevent undesirablevibrations in said member when the coil is v1- brated.

26. An acoustic device rectly to theisurrounding air, means attached tosaid diaphragm at points removed from the center thereof for driving it,means to prevent the sound waves radiated by the exposed face of thediaphragm from interfercomprising a diaphragm having one face thereofexposed diing with those radiated by the other face of said diaphragm,and means for damp 27. An acoustic device comprising a casing havin afront wall with an aperture therevin, si e and back walls, a magnetmounted within said casing having annular pole pieces spaced toform anannular air gap, a diaphragm comprising a curvedportlon and asurrounding annular portion, a coil in said air gap secured to thediaphragm at the edge of the curved portion thereof, means for anporting the outer rim of said diaphragm 1n the waves given off by saidother face of the diathe aperture in said casing so that it is to movesubstantially as a whole, and means within said casing and surroundingsaid magnet for damping sound waves "given of b one side of thediaphragm 28. A loud speaking device comprising 3,' dished diaphragmhaving one me thereof. exposed directly to the surrounding air and ofsufficient size to radiate sound directly into the surrounding airwithout the emplo ent of a horn, means including a coil a e'd'to said dihragm at dgqiuts removed from the thereof for s eems' 1m an apertureinto which said dia hragm fits, and means with-in said casing or dampinthe sound waves given off by the unexpose face of said diaphragm.

29. The combination of a diaphragm for radiating sound waves directlyinto the surrounding air without the employment of a horn, meansincluding a coil attached to said diaphragm and a magnet structure fordriving it, a casing having an aperture in one wall thereof in whichsaid diaphragm is disposed and a filling of sound deadening material insaid casing and surrounding said magnet structure.

In witness whereof, I hereunto subscribe my name this th da of March A.D. 1925. ED ARD c. wENTE.

so i

